Outside the real world, there is an abnormal space called "zero space" composed of human memory, choices and unfinished obsessions.
It will select specific humans through black cards and bring them into "realms" that violate the laws of reality. Candidates must observe the environment, understand the rules, and use limited card abilities to complete tasks.
The loser may die, lose his memory, or be left in the realm and become a character in the next round of the game.
The core of the story is not simply fighting monsters, but:
Determining what to believe when information is incomplete and rules may be misleading.
